1. Field of the Invention
The present invention relates to a hybrid work machine including an engine, an assist motor, a hydraulic pump which is driven by total torque of the engine and the assist motor, and an electric storage device which accumulates electric power generated by the assist motor and supplies the electric power when the assist motor performs electric discharge.
2. Description of the Related Art
In recent years, in automobiles, the hybrid type and the electric type are becoming prevalent in terms of energy saving, and the hybridization is being promoted also in work machines such as construction machines. In general, a work machine such as a hydraulic excavator driven by a hydraulic system includes a hydraulic pump which enables work at the maximum load and a large-sized engine which drives the hydraulic pump, so that the work machine can deal with every type of work from low load work to heavy load work.
However, the heavy load work, such as heavy excavation work in which the hydraulic excavator frequently performs excavation and loading of earth and sand, is only a part of the entire work, and the high ability of the large-sized engine becomes surplus at times of the light load work such as the leveling for smoothing the ground surface. This is one factor that impedes the reduction of the fuel consumption of hydraulic excavators. In consideration of this point, a hybrid work machine, in which the engine is downsized for the reduction of the fuel consumption and insufficient output power due to the downsizing of the engine is supplemented (assisted) by output power produced by an electric storage device and an assist motor (electric motor) as described in JP-2012-172521-A, for example, have been proposed and have become widely known.
Further, in the technology of JP-2012-172521-A, a characteristic in regard to the setting of pump maximum input power is switched to one of first and second two characteristics depending on whether the hydraulic pump is in a normal operation state in which the hydraulic pump delivers hydraulic fluid from a low pressure range to an intermediate range or in a high load pressure operation state in which the hydraulic pump continuously delivers hydraulic fluid in a high pressure range.